After using the Mac for a few months now, I have two tips that will speed up the performance of your shiny new machine.

Disable Dashboard. It's a dog. It doesn't make so much of a difference on a Mac Pro, but my MacBook Pro was really feeling it.

Disable the patented "genie effect" on Mac OS X. All that window zoomy zoomy is fun and all, but it makes the OS feel sluggish. When I disabled the effects on my MBP, suddenly everything felt a lot faster, even things that I wouldn't have associated with high-overhead graphics. For example, it vastly improved the response time when running rules in Mail using MailTags and Mail Act-On.

Oh, and by "a lot faster", I mean that is still feels a lot slower than on my old Thinkpad. But that's ok. It's prettier. And I do like many features of the OS more.